Nonvolatile memory devices retain information stored thereon when power is removed from the memory device. Nonvolatile memory devices may be classified as, for example, read-only memory (ROM), erasable and programmable ROM (PROM), and electrically erasable and programmable ROM (EEPROM). EEPROMs are typically configured to allow modification of information quickly and easily relative to ROMs or EPROMs. One type of EEPROM is a flash memory device, which is typically highly integrated. Thus, the demand for flash memory devices is rapidly increasing.
Flash memory devices may be classified, for example, as floating-gate and charge-trap types according to a type of charge-storing structure. In particular, a floating-gate flash memory, which is a general type of flash memory, includes unit cells each having sequentially stacked floating and control gate electrodes. Floating-gate flash memory devices may experience technical problems when highly integrated. For example, highly integrated flash memory devices may have increased aspect ratios of gate patterns, increased aspect ratios of gap regions between gate patterns, and increased interference between floating gate electrodes.
Charge-trap flash memory devices are similar in structure to a conventional metal oxide semiconductor field effect transistor (MOSFET) because it uses a film having a plurality of trap sites, such as charge trap layers, instead of the floating gate. Furthermore, as the charge trap films are formed to have thicknesses less than a thickness of the floating gate electrode, the charge-trap flash memory devices may not experience the same problems discussed above with respect to the floating-gate flash memory devices. Thus, charge-trap flash memory devices are becoming more widely used and studied. Charge-trap flash memory devices may have silicon-oxide-nitride-oxide-semiconductor (SONOS) or metal-oxide-nitride-oxide-semiconductor (MONOS) structures using silicon nitride for the charge trap film.
According to recent studies with respect to charge-trap flash memories, however, it has been discovered that charge could spread laterally in a silicon nitride film. Since the condition of data stored in a memory cell is typically dependent on a density of charges trapped in the silicon nitride film, such a lateral spread may degrade retention characteristics of the charge-trap flash memory. Thus, techniques for reducing the likelihood that charges will laterally spread by separating the charge trap film forming each memory cell have been proposed.